1. Field of Invention
This invention relates to instrumentation and particularly to instrumentation and control in an environment requiring accurate and continuous monitoring during operation. In particular, this invention relates to an apparatus for switching high current inductive loads while monitoring the status of load circuitry. A particular application of this invention is in the drive of an AC controlled field solenoid connected at a distance of up to about one-half mile from the control source as in the environment of an operating nuclear power plant. The invention has other applications where relatively high current levels are controlled by standard logic level signals and where mechanical contact relays are insufficiently reliable in the operational environment.
Certain control functions require the use of relatively high level AC current in circuits having wire lengths of up to one mile or more. The relatively lengthy, high current circuits present special control problems. For example, the lengthy circuits are subject to unwanted high voltage and high current transients due both to the external environment and to the inductance in the circuit. There is nevertheless, the need to measure accurately the current in the driven circuit from a remote location.
2. Description of the Prior Art
In the past, high current load driving circuits have been controlled with low level logic signals through the use of DC circuitry and mechanical contact relay circuits. Reliability and noise immunity are significant concerns in such designs. Moreover, mechanical relays inherently have only a finite life. Frequent testing of a relay circuit causes the relay contacts to degrade, thereby degrading the reliability of the circuit. In addition, testing procedures are such that a relay must be removed from its normal circuit for testing to avoid effect on the load. Degradation caused by self-testing is generally unacceptable in an environment requiring high reliability.